Black holes are one of the most mysterious and terrifying phenomena to populate the universe, and according to a new study, there may be far more of them than we realize: A total of 40 quadrillion (40,000,000,000,000,000).
The findings come in the first study of a series published in the peer-reviewed academic periodical The Astrophysical Journal, which focuses on modeling the mass function of black holes of various sizes, ranging from stellar all the way to supermassive.
How many black holes are there in the universe?
That's a difficult question and likely cannot be answered any time soon. But that is not the case with the observable universe, a vast expanse of space that has a diameter of approximately 90 billion light-years that represents the furthest extent of what in the universe that can be seen by us on Earth with all the telescopes, space probes and other methods of observation currently at our disposal.
So how many black holes are there in the observable universe?
According to the calculations of this study, that's 40 quadrillion, also known in scientific notation as 40 x 10^18.
But that's not all.
All these black holes gather a lot of matter into them through accretion. But just how much matter is that?
According to the study, black holes have taken up around 1% of all ordinary matter. This specifically refers to baryonic matter, which would describe most things encountered in everyday life. Non-baryonic matter refers to things like free electrons, neutrinos and, most notably, black holes.
How do we know how many black holes exist?
In short, scientists did the math.
In order to determine the number of black holes in the observable universe and how much matter they have taken in, the researchers developed a new method of calculation. This was done by evaluating a number of factors like rate of star formation, the amount of stellar mass and metallicity - the abundance of metals, which, in astronomy, refers to any element heavier than hydrogen and helium.
It further relied on the use of a stellar and binary evolution code to further calculate the evolution of stars. This evolution code is combined with data and information about star formation and the enrichment and distribution of metals in individual galaxies.
Then, it was all put into a model alongside the use of necessary equations.
It's complicated, to say the least. But it is also innovative; a robust and revolutionary "computation of the stellar black hole mass function across cosmic history," lead author PhD student Alex Sicilia at the International School for Advanced Studies (SISSA) in Italy noted in a statement.And its complexity is only further heightened by how multidisciplinary it is. As noted by Sicilia's supervisor Prof. Andrea Lapi, it required expertise in galaxy formation and evolution, gravitational waves, stellar astrophysics and more.
What are black holes?
Black holes are, to put it simply, massive concentrations of gravity that are so strong, nothing, not even light, can escape them. As such, it can be very hard to spot them. In fact, scientists weren't even sure they existed 20-30 years ago. The only way we know black holes exist is because they have an enormous gravitational pull that influences surrounding matter. In other words, it can be hard to see a black hole, but we can see it affecting everything around it.
Most black holes are formed when a star dies, its core collapsing in on itself to form one massive concentration of gravity. They continue to accrete matter over time and, while light and matter can't escape, radiation and radio waves do get emitted.
Today, our scientific understanding of black holes as grown considerably, and we now know a lot more about them. For example, supermassive black holes – which, as their name implies, are absolutely enormous – can be located at the centers of most major spiral galaxies. Our own Milky Way galaxy is no exception, with the supermassive black hole Sagittarius A* being located in the galactic center.
The recent study may have also shed light on another intergalactic mystery: The origin of supermassive black holes.
How are supermassive black holes created?
While the origins of most black holes are known, how supermassive black holes are created is a lot more unclear.
It is thought that black holes gather material rapidly from the surrounding environment and are surrounded by what is known as an "accretion disk." These disks consist of fast-rotating high-temperature gases that give off light - though the black holes themselves do not.
However, it is unclear if this is true, and our understanding of this process may be very much incomplete.
This is something many scientists, including an international research team led by the University of Haifa utilizing NASA's Hubble Space Telescope, are trying to answer.
But this study presents a theory.
To put it simply, supermassive black holes are thought to form with a seed, which has been hypothesized to be formed when galaxies that birth stars and experience supernovae see gases migrate inward to the galactic center which will, eventually, fully condense and merge until a supermassive black hole seed is formed. It is then that the black hole grows bigger through accretion.
That much was already known, but what the researchers figured out was about seed distribution. Light seeds are distributed throughout the universe, but how do they become heavier? That seems to be through the mergers of star clusters and collapses, which would form heavier seeds. These heavier seeds in turn are what are used to grow these supermassive black holes.
Right now, this is very much theoretical, but it is something the team is keen to focus on next in an upcoming study. But it brings us another step closer to understanding the mysteries of black holes and how they influence and impact the universe around them.